Automotive wire located under the hood in the engine compartment (engine wire) has traditionally been insulated with a single layer of high-temperature insulation that is disposed over an uncoated copper-wire core. The U.S. specification that is usually used for this wire is SAE J1128, rev. January 1995 type TXL. The high temperature requirements for type TXL insulation, which require specific tensile and elongation standards as well, specifies that the insulation be oven aged without the conductor at 155.degree. C. for 168 hours.
For certain newer automobiles, the high temperature specified for type TXL insulation is not sufficient to cover the actual high temperatures existing in automobile engine compartments. The new requirements for engine wire range from135.degree. C. to 180.degree. C. A typical test procedure now required is aging the insulation with the conductor for 3,000 hours at 150.degree. C. as specified in International Standard ISO 6722-1 and 6722-2, rev. January 1996.
Most of the high-temperature wire used as engine wire in North America uses cross-linked polyethylenes (XLPE) as the insulation material. European manufacturers have obtained good high-temperature performance (3,000 hrs. at 155.degree. C.) using thermoplastic polyesters. This insulation has outstanding resistance to gas and oil, is mechanically tough, and resists copper catalyzed degradation. Thermoplastic polyesters, however, can prematurely fail, because of hydrolysis. Thermoplastic polyester insulated wires have also been found to crack when exposed to hot salty water. Thermoplastic polyesters have also failed temperature humidity cycling as specified in the United States Car Specification PF-9600, Change A. As a result of these weaknesses, the use of thermoplastic polyester insulation has been limited to uses other than under-the-hood applications in North America.
The XLPE insulation, on the other hand, has most of the desired properties of the polyester materials, in addition to possessing good resistance to water. However, this material degrades when it comes into contact with copper at high temperatures.
It is possible to tin coat the copper core in order to prevent the copper from contacting the XLPE, but the additional cost of tin and the tin coating process are expensive. In addition, most automotive specifications require that the copper core be uncoated.
It is also possible to add copper stabilizers to the polyethylene compound, but copper stabilizers (such as Ciba Geigy Irgonax MD-1024) yield only partial protection for wire having thin wall thicknesses, when used at 150.degree. C.
The amount of wiring in automobiles has increased exponentially, as more electronics are being used in modern vehicles. This dramatic increase in wiring has motivated automobile manufacturers to reduce overall wire diameter by specifying thinner wall thicknesses, and specifying smaller conductor sizes. The forthcoming ISO 6722-5 specification (referred to as ultra-thin wire) reduces the insulation wall thickness to 0.20 mm. One automobile manufacturer in North America has released a new specification requiring a 0.15 mm wall thickness.
These reductions in insulation wall thicknesses pose manufacturing difficulties for wire fabricators. For XLPE, the thinner wall thickness of the insulation results in shorter thermal life, when aged at oven temperatures between 150.degree. C. and 180.degree. C. This limits their thermal rating. For example, a copper wire with an XLPE insulation having a 0.75 mm wall thickness is flexible, and does not crack when bent around a mandrel after being exposed to 150.degree. C. for 3,000 hours. But this same copper wire, with the same XLPE insulation having a 0.25 mm wall thickness, becomes brittle after being exposed to 150.degree. C. for 3,000 hours.
The deleterious effects created by these extremely thin wall requirements have been attributed to copper catalyzed degradation, which is widely recognized as a problem in the industry.
The present invention reflects the discovery that the desired performance of the insulation can be achieved by utilizing an integrated composite of insulation layers, rather than a single layer. The composite comprises an inner layer of fluorocarbon polymer and an outer layer of cross-linked polyethylene material. The inner layer of fluorocarbon polymer is designed to resist copper catalyzed degradation, while the outer layer of cross-linked polyolefin material is designed to be chemically resistant to automotive fluids and salt water, as well as to resist high temperatures. The outer layer is also designed to provide the required mechanical protection. This composite insulation makes the wire suitable for under-the-hood automotive applications.
Other high temperature polymers can be used as an inner layer to prevent copper migration into the XLPE. Such high temperature polymer can be selected from a group consisting of: polyether sulfone, poly-ether-ketone, polyetherimide, and thermoplastic polyester. These polymers can be used in place of a fluorocarbon polymer. A third optional adhesive layer could form the composite to bond the inner and outer polymer layers.
Considering that current automotive specifications call for only one layer, a manufacturer would not be inclined to combine several layers, nor have an expectation of obtaining the desired characteristics by employing several layers. The XLPE outer layer can be chemically cross-linked or can be cross-linked by irradiation processing. The irradiation process of cross-linking the polyethylene suggests that certain fluorocarbon homopolymers made from monomers such tetrafluoroethylene and hexafluoro-propylene should not be used. This is because their properties are adversely affected by irradiation. However, fluorocarbon copolymers such as ethylene-tetrafluoroethylene can be irradiation cross-linked with a reactive monomer, such as triallyl isocyanurate. The triallyl isocyanurate increases the high temperature capability.